The present invention relates to improved methods and apparatus for determination of the total concentration of organic carbon compounds in aqueous process streams. The invention is especially adapted for use in measuring carbon in deionized water or deionized water with dissolved carbon dioxide, which is often used in research and development laboratories, and in manufacture and processing of electronic components, fine chemicals and pharmaceuticals.
Particularly, the method of the present invention in a preferred embodiment includes oxidation of organic components of a sample stream of water and measurement of electrical conductance and/or electrical resistance of such stream thereafter (preferably also the temperature of such stream). The preferred embodiment requires no pump or flow control components other than an on/off valve (e.g., a solenoid valve), making it inexpensive and reliable. Further, the cell containing the conductivity electrodes and temperature sensor is removable from other components of the sensor for maintenance or calibration when required.
Measurement of Total Organic Carbon (TOC) is a well-established method of determining the concentration of organic contaminants in water [e.g., Van Hall, C. E.; Safranko, J. and Stenger, V. A., xe2x80x9cRapid Combustion Method for the Determination of Organic Substances in Aqueous Solutions,xe2x80x9d Anal. Chem., Vol. 35(3), pp. 315-319; 1963; also, Poirier, S. J. and Wood, J. H., xe2x80x9cA New Approach to the Measurement of Organic Carbon,xe2x80x9d Am. Laboratory, pp. 1-7; December 1978.]. In all TOC measurement techniques, carbon in the organic contaminants is oxidized to carbon dioxide, which is measured by a variety of means.
Water may already contain carbon dioxide and other inorganic sources of carbon, so it is necessary to either eliminate Inorganic Carbon (IC) from the sample, or measure IC, prior to TOC measurement. In those techniques that measure IC, carbon dioxide concentration, after oxidation of the organics, is the sum of carbon dioxide from organic and inorganic sourcesxe2x80x94Total Carbon (TC). TOC concentration is calculated from the difference between TC and IC. (As used herein, xe2x80x9ccarbon dioxidexe2x80x9d is intended to include free carbon dioxide, carbonic acid and bicarbonate.)
A variety of prior art approaches for measuring TOC content of water have been proposed. The so-called xe2x80x9cdifferential conductivityxe2x80x9d technique involves measurement of conductivity and temperature before and after the oxidation of the organic components of the water sample. Oxidation is initiated by action of ultraviolet (UV) light on the sample water.
The differential conductivity technique was implemented using either a continuously flowing sample stream [e.g., Bender, D. and Bevilacqua, A. C., xe2x80x9cPortable Continuous TOC Monitoring in a Semiconductor Water System,xe2x80x9d Ultrapure Water, pp. 58-67; October 1999; see also U.S. Pat. No. 4,749,657] or a sample stream that operates in a stop-flow mode [e.g., Gallegos, P.; Stillian, J.; and Rasmussen, J., xe2x80x9cConductometrically Based TOC Detection Instrumentation""s Accuracy in Semiconductor High-Purity Water,xe2x80x9d Proceedings of Watertech ""99 Executive Forum, pp. P1-P21; Portland, Oreg.; Oct. 5-6,1999; see also U.S. Pat. Nos. 4,868,127; 5,275,957; and 5,677,190].
Problems with the Continuous-Flow Approach
The continuous-flow approach is usually used where fast detection of rapidly changing TOC concentrations is desired. At least two conductivity cells are usually used. Water flows continuously through a first conductivity cell, then the UV reactor, and finally the second conductivity cell. The difference in temperature-compensated conductivity measurements between the two cells is used to calculate the response of the analyzer.
The flow rate of water is necessarily rapid to achieve this rapid response. To be economically competitive, the UV reactors are made too small at those flow rates for the UV exposure to oxidize organics all the way to carbon dioxide. Instead, organic acids are formed, and these acids increase conductivity of water more than carbon dioxide does at the same concentration. This produces a large positive error in measurement of TOC. To overcome this problem, those analyzers must be calibrated with solutions that contain organic compounds thought to be similar to those in the unknown water sample. Since composition of all process waters actually changes with time, these analyzers do not accurately report TOC values.
A second problem with continuous-flow techniques is that they require the flow rate be held constant, even when water pressure changes. Otherwise, already quite inaccurate measurements become unusable. This necessitates addition of relatively expensive and complex flow- and pressure-control devices to those analyzers. The analyzers also require the operator to monitor flow rate of the sample, and to make periodic manual adjustments to the flow.
Problems with the Stop-Flow Approach
In the stop-flow approach, measurements are not affected by variations in water pressure, so flow- and pressure-control devices are not required. This is because conductivity electrodes are placed inside the UV reactor, allowing conductivity measurements to be made while water is stopped in the UV reactor. Only a solenoid valve or similar device is required to stop sample flow.
Conductivity and temperature of the water are measured, then flow is stopped, the UV lamp is turned on and oxidizes organics. When oxidation is thought to be complete, conductivity and temperature of the solution are measured again. TOC concentration is calculated from the difference in temperature-compensated conductivity measurements made before and after oxidation. Then the solenoid valve is opened to allow the UV reactor to be flushed out with a fresh water sample, in preparation for the next measurement.
A problem with the stop-flow approach stems from the fact that the electrodes are in the UV reactor. The UV lamp illuminates the electrodes during the oxidation period, and they generally are coated with a photocatalytic material, such as titanium dioxide. When illuminated, this material catalyzes oxidation of organics. The problem is that some organics are converted to organic acids and later are further oxidized to carbon dioxide. Such organic acids initially make the water more conductive than it will be later when the acids are converted to carbon dioxide. This means conductivity peaks at a very high level, and then decreases as acids are converted to carbon dioxide. Conductivity asymptotically approaches a steady-state value as all of the acids are converted to carbon dioxide. If conductivity is measured too soon, a positive error results in TOC measurement.
To avoid this mistake, some stop-flow TOC analyzers make repetitive conductivity and temperature measurements. Complex algorithms must be used to monitor temperature-compensated conductivity to detect conductivity peaks. When a peak is detected, the analyzer must track the subsequent decrease in conductivity to estimate its steady-state value. This requirement increases the time for the measurement, and makes it impossible to predict how long each measurement will require. Additionally, the algorithm increases the complexity of the analyzer and cost of its electronics.
A second problem is that, because the conductivity electrodes are placed inside the UV reactor, they cannot be removed easily for maintenance or calibration unless the UV reactor is constructed with multiple parts, including seals around the conductivity electrodes. This complexity makes the UV reactor larger, increases cost, and decreases reliability.
A third problem is that stop-flow TOC analyzers are subject to errors in the conductivity measurement used to calculate the IC concentration. Hydrogen peroxide is known to be formed by electrodes in contact with water containing dissolved oxygen, even in the absence of UV light [e.g., Clechet, P.; Martelet, C.; Martin, J. R. and Olier R., xe2x80x9cPhotoelectrochemical Behaviour of TiO2 and Formation of Hydrogen Peroxide,xe2x80x9d Electrochimica Acta, Vol. 24, pp. 457-461; 1979]. Hydrogen peroxide decomposes to oxygen by a reaction mechanism that is catalyzed by metals (i.e., conductivity electrodes). This catalyzed decomposition proceeds through formation of highly reactive hydroxyl radicals [e.g., Cotton, F. A. and Wilkinson, G., Advanced Inorganic Chemistry, Second Ed.; Interscience Publishers, New York, N.Y., pp. 374; 1966]. Hydroxyl radicals react with organics in the UV reactor before the UV lamp is turned on, forming acids and other species that produce highly conductive ions in solution. Thus, the IC conductivity measurement can have significant positive errors, which cause low errors in TOC concentration calculation.
A fourth problem exists. Even in the absence of reactions at the surface of the electrodes, described above, conductivity electrodes and other UV reactor materials leach ions into the water. These ions increase conductivity of water, and they cause errors in both IC and TC measurements.
A fifth problem is that, in the presence of UV light and dissolved oxygen in the water sample, the TOC measurement exhibits a positive error due to formation of hydrogen peroxide. Dissociation of hydrogen peroxide itself results in an increase in conductivity of water [Gallegos, P.; Stillian, J.; and Blades, R., xe2x80x9cLight Dependent Compensation of TOC Measurement and its Relationship with Dissolved Oxygen Concentrations in Ultrapure Rinse Water for Semiconductor Manufacturing,xe2x80x9d presented at 18th Annual Semiconductor Pure Water and Chemicals Conference in Santa Clara, Calif.; Mar. 1-4, 1999.]
Other prior art approaches to measuring TOC content of water include U.S. Pat. Nos. 3,958,941 of Regan; 4,749,657 of Takahashi; 5,518,608 of Chubachi; 4,868,127 and 5,275,957 of Blades; 5,677,790 of Melanson; and 5,272,091 of Egozy.
In U.S. Pat. No. 3,958,941 of Regan, an aqueous sample is introduced into a circulating water stream that flows through a reaction chamber where the sample is mixed with air and exposed to UV radiation to promote oxidation of organic compounds found in the sample to form carbon dioxide. Free carbon dioxide formed in the reaction chamber is then removed from solution by an air stripping system and introduced into a second chamber containing water that has been purified to remove ionic compounds. Conductivity of water in the second chamber is measured, and any increase in conductivity is related to the total concentration of carbon dioxide following oxidation in the first reactor. The conductivity measurement can be used, therefore, to determine the concentration of total carbon in the original sample. If the concentration of inorganic carbon is known, or if inorganic carbon compounds are removed prior to the measurement, the concentration of organic carbon can be determined.
In U.S. Pat. No. 4,749,657 of Takahashi, sample flows continuously through a UV light reaction zone, in the presence of oxygen, and is partially oxidized (but not completely) to carbon dioxide. Conductivity of the partially oxidized organics is measured in a conductivity cell. An inlet conductivity cell also may be employed to determine the original conductivity of the solution.
In U.S. Pat. No. 5,518,608 of Chubachi, a plant for producing and using ultrapure water is described, consisting of a first TOC monitor, a water purifier containing ion exchange resins, and a second TOC monitor. TOC concentration in the purifier output water is continuously monitored to allow early detection of when the ion exchange resin is deteriorated. The xe2x80x9cTOC monitorsxe2x80x9d referred to are identified as resistivity sensors elsewhere in the patent.
In U.S. Pat. Nos. 4,868,127 and 5,275,957 of Blades, a water sample is introduced into a sample cell, the body of which is constructed from Teflon or ceramic. The cell contains a quartz window, sealed with an O-ring. On the other side of the quartz window is a housing that contains a UV lamp. Two concentric circular electrodes are located inside the cell, and are constructed of titanium, palladium, iridium, rhodium or platinum. A temperature sensor is in contact with one of the electrodes. Temperature-compensated conductivity measurements are made before, during and after oxidation of organic compounds in the water sample. When oxidation is complete, the sample water is allowed to flush the cell out prior to the next measurement.
In U.S. Pat. No. 5,677,790 of Melanson, a measurement cell and circuitry are described. The cell is constructed from quartz or glass tubing, and a pair of wire electrodes is positioned longitudinally inside the cell. The electrodes are made from titanium and have catalytic titanium dioxide surfaces. A temperature sensor is mounted outside, but in contact with, the measurement cell wall. A UV lamp is positioned to irradiate the measurement cell and the electrodes inside it. The circuit used to monitor temperature-compensated conductivity of the solution in the measurement cell multiplexes a drive signal between the conductivity electrodes, a calibration resistor, and the temperature sensor.
In U.S. Pat. No. 5,272,091 of Egozy, water passes through a resistivity cell and a three-way valve. In one position, the three-way valve directs water into an oxidation zone (UV reactor) and a second resistivity cell. In the other position, the three-way valve bypasses flow of water away from the oxidation zone. This allows a portion of the water to remain stagnant in the oxidation zone for any desired time. When the three-way valve next directs flow into the oxidation zone, water that has remained there during the oxidation period now flows through the second resistivity cell. This allows the resistivity of the oxidized water to be measured for a variety of oxidation periods. Resistivity that would be measured after infinitely long oxidation periods can be estimated. That estimate is then used to calculate TOC concentration in the water.
Disadvantages of Former Methods
The Regan device is slow, cannot be used for continuous monitoring of TOC concentration in aqueous streams, cannot be scaled down without increasing interference from NO2, SO2, and H2S to unacceptable levels, and is generally unsatisfactory. In any system that requires removal of carbon dioxide from an aqueous solution by air stripping, pH of the solution must be reduced to 4.0 or less to ensure that all carbon is in the form of free carbon dioxide. The Regan patent does not teach that acid must be added to the sample stream to achieve this requirement.
In the method of Takahashi, UV light, in the presence of oxygen, only partially oxidizes organic compounds. Carbon dioxide is not produced as the end product of oxidation. The products of oxidation are indeterminate, and vary with time as the organic compounds in the influent water vary. Even when composition of influent water does not change, oxidation products change with water flow rate, temperature, and other operating parameters. This is because these parameters change the degree to which the original compounds in the water are oxidized. This produces inaccurate TOC measurements because conductivity of compounds produced in the oxidation differ from one another. In an attempt to control operating parameters, expensive flow- and pressure-control components must be added. The operator also must make adjustments to these devices periodically, which increases operating costs of the Takahashi method.
The Chubachi device has the same disadvantages as the Takahashi invention.
The Blades devices and the Melanson device have several disadvantages. In these devices, conductivity electrodes are located in the UV reactor and are covered with a photocatalyst. Because photocatalyzed reactions occur at the electrode surfaces, conductivity measurements are confounded by initial production of organic acids. These acids are produced before some organics are completely oxidized to carbon dioxide. The result is that the acids produce an excessively high conductivity reading, which can be mistaken for a very large TOC concentration. If this error is to be avoided, a complex algorithm must be used to detect conductivity peaks. This algorithm extends the length of the oxidation period to estimate steady-state conductivity that will be achieved when acids are completely oxidized to carbon dioxide. This algorithm increases the complexity of the analyzer, increases cost of the electronics, and makes the duration required for each measurement unpredictable.
Locating and operating conductivity electrodes in the UV reactor also produces hydrogen peroxide that can oxidize organics, even before the UV lamp is turned on. Acids produced in this oxidation increase conductivity of the water while initial conductivity measurements are made to determine IC concentration. This causes a positive error in IC measurement and a negative error in TOC measurement.
Having electrodes and other materials, other than quartz, in the UV reactor also produces errors due to leaching of ions into water. These ions increase conductivity of water, causing errors in measurement of both IC and TC.
In the method of Egozy, resistivity, following oxidation for an infinitely long period, is estimated from several measurements made for several different oxidation periods. Projected resistivity for infinitely long oxidation periods is then used to calculate TOC of the solution. One disadvantage of this method is that the TOC calculation is performed only after several measurements are made, making the method very slow. Another disadvantage is that projection of resistivity for infinitely long oxidation periods cannot be made if composition of water samples change while measurements are being made. Therefore, this method cannot be used on process streams that even change their composition very slowly.
Advantages of the Invention
The present invention has none of the problems of known continuous-flow and stop-flow TOC analyzers, and all of their advantages. Unlike the continuous-flow approach, a small, inexpensive UV reactor can be used to completely oxidize organics to carbon dioxide because the time that the water sample is exposed to UV light is controlled by operation of a solenoid valve. No expensive flow- or pressure-control components are required, and no operator intervention is needed to maintain a fixed flow rate.
The present invention also has better accuracy than the stop-flow approach because conductivity electrodes are located outside the UV reactor, where UV light cannot illuminate them. This avoids any possibility of unwanted photochemical reactions. The UV reactor is constructed entirely of quartz to eliminate leaching of extraneous ions into the portion of the sample in which organics are to be oxidized. Furthermore, in a preferred embodiment, pulsed flow of sample water occurs before conductivity measurements begin. This ensures that any ions leached from conductivity electrodes are swept away from the electrodes prior to the measurements.
Such preferred embodiment additionally has the advantage that its design can be implemented using unusually inexpensive manufacturing techniques that reduce material costs and assembly labor, and increase reliability of the instrument. Such preferred embodiment has an advantage related to maintainability, because the conductivity cell is removable from the UV reactor. Either part can be easily removed for maintenance or replacement when necessary.
In its broadest embodiment, the apparatus of the present invention comprises:
an organic carbon irradiation chamber having a fluid inlet conduit and a fluid exit conduit, wherein at least part of said chamber is substantially permeable to radiation capable of generating radiation-initiated products of said organic carbon,
said fluid exit conduit being in fluid communication with measuring means effective to measure one or more of said products,
means to provide a stream of said water, and
means effective to provide seriatim the following steps:
(a) flowing said stream seriatim through said inlet conduit, said chamber, said exit conduit and said measuring means, thereby at least partly filling said chamber with said water;
(b) stopping flow of said stream, whereby said chamber remains at least partly filled with said water;
(c) irradiating said water in said chamber with said radiation for a first predeterminable time and temperature effective to produce products of said organic carbon in said water, thereby producing irradiated water;
(d) flowing at least part of said irradiated water into said measuring means;
(e) stopping flow of said irradiated water into said measuring means, whereby said measuring means retains part of said irradiated water
(f) effecting a measure of one or more of said products in said measuring means, thereby producing a first measure of said products;
said apparatus further comprising one or more additional means selected from the group consisting of:
A. estimating means for estimating said organic carbon content at least in part from said first measure, thereby producing an estimate of said organic carbon content
B. displaying and/or recording means for displaying an estimate of said organic carbon content.
C. first measure comparing means for comparing said first measure with a predeterminable value of said first measure.
D. logic means for estimating said organic carbon content at least in part from said first measure, thereby producing an estimate of said organic carbon content, and for comparing said estimate with a predeterminable value of said estimate
E. irradiating means for irradiating said water in said chamber with radiation effective to react said organic carbon substantially to carbon dioxide, carbonic acid and/or bicarbonate, and to predetermine said first predeterminable time and temperature at values sufficient to react said organic carbon substantially to carbon dioxide, carbonic acid and/or bicarbonate as products of said organic carbon in said water.
F. shielding means effective substantially to shield said measuring means from said radiation
G. temperature and electrical conductivity and/or electrical resistance detecting means in said measuring means
H. temperature and electrical conductivity and/or electrical resistivity detecting means in said measuring means, said detecting means comprising stainless steel electrodes immersible in fluid in said measuring means,
I. irradiating means for irradiating said water in said chamber with radiation including wavelengths less than and equal to 254 nanometers,
J. irradiating means for irradiating said water in said chamber with radiation including wavelengths in the range of from about 160 nanometers to about 200 nanometers, and,
K. stepping means to provide seriatim steps including a step flowing said stream seriatim through said inlet conduit, said chamber, said exit conduit and said measuring means, thereby at least partly filling said chamber with said water; a step stopping flow of said stream whereby said chamber remains at least partly filled with said water; a step retaining water from said last mentioned step in said chamber for a second predeterminable time and temperature in the absence of irradiating said water with said radiation; a step flowing at least part of water retained in said chamber in said last mentioned step into said measuring means and stopping flow of said water, whereby said measuring means retains part of said water; a step effecting a measure of said one or more of said products in water retained in said measuring means in said last mentioned step.
In its broadest embodiment, the process of the present invention comprises:
estimating organic carbon content of a stream of water using an organic carbon irradiation chamber having a fluid inlet conduit and a fluid exit conduit, wherein at least part of said chamber between said inlet and outlet conduits is substantially permeable to radiation capable of generating radiation-initiated products of said organic carbon, said fluid exit conduit being in fluid communication with a measurement system to measure one or more of said radiation-initiated products or said organic carbon, the process comprising seriatim the following steps:
(a) flowing said stream seriatim through said fluid inlet conduit, said irradiation chamber, said fluid exit conduit, and said measurement system, thereby flushing said conduits, said chamber and said measurement system, and at least partly filling said chamber with said water;
(b) stopping flow of said stream, whereby said chamber remains at least partly filled with said water;
(c) irradiating said water in said chamber with said radiation at least through said part of said chamber substantially permeable to said radiation for a first predeterminable time and temperature to generate radiation-initiated products of organic carbon in said water in said chamber, thereby producing irradiated water,
(d) flowing at least part of said irradiated water into said measurement system;
(e) stopping flow of said irradiated water into said measurement system, whereby part of said irradiated water is retained in said measurement system;
(f) measuring said one or more of said radiation-initiated products of said organic carbon in said part of said irradiated water, thereby producing a first measure of said radiation-initiated products;
(g) estimating said organic carbon content at least in part from said first measure, thereby producing an estimate of said organic carbon content; and,
(h) transmitting by means of electrical signal and/or displaying and/or recording said estimate.
The process of this invention may further comprise the additional steps of:
(i) retaining said water from step (b) in said chamber without irradiating said water with said radiation for a second predeterminable time and temperature, thereby producing non-irradiated water;
(j) flowing at least part of said non-irradiated water from step (i) into said measurement system;
(k) stopping flow of said non-irradiated water into said measurement system, whereby part of said irradiated water is retained in said measurement system;
(l) measuring said one or more of said radiation-innitiated products of said organic carbon in said part of said non-irradiated water, thereby producing a second measure of said radiation-initiated products;
(m) estimating said organic carbon content at least in part from said first measure and said second measure, thereby producing an alternative estimate of said organic carbon content; and,
(n) transmitting by means of electrical signal and/or displaying and/or recording said alternative estimate.